Coquina from the Quaternary of Florida, USA

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Limestone is a common biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). The most distinctive type can be given the generalized name fossiliferous limestone, which is composed of abundant whole & fragmented fossil shells and skeletons, usually mixed with mud. Most limestones represent deposition in ancient warm, shallow ocean environments.

The sample shown above is a distinctive variety of fossiliferous limestone called coquina. It is composed of finely-busted up seashells, typically clam shells & snail shells (bivalves & gastropods). Coquinas have high porosity - a significant amount of empty space exists between the grains. This results in coquina having a spongy appearance.